ok, just making shit up, but using numbers so we can get an estimate of what you have
Mine might be low as you said your bro bought a motorcycle
but Suppose the purchase was $300 in 1978... about 1.55 ounces.
current value: approximately $6,600
now if you had like $3,000 in 78 (unlikely), would be worth 66,000
Sidefi advice
I'd split into 2 equal amounts
1. sell today as that's a lot of appreciation and likely will never grow that much in the next 15-20 years and and split it up in a few EFTs (AIQ, DJI, QQQ, XLE or XLU) and maybe a little crypto (stick with BTC/ETH) for diversity
2. Put in a trust in your kids name and tell them not to touch in until they are 65 ...as it will probably be worth a lot then
or just do #2 with all of it but say 1 trust they have when they are 25 and one when they are 65
but honestly, there is nothing stopping them from getting access to it the day after you set it up
OR you could do the exact opposite (not sure what that would be, buying scratch off tickets with the proceeds?)